1. Field of the Invention
The present invention relates to a discharge lamp lighting device which, as a backlight source for a large liquid crystal display device, lights a plurality of discharge lamps.
2. Description of the Related Art
A discharge lamp lighting device with a high-frequency lighting circuit has been proposed which lights cold cathode discharge lamps as a backlight source for a large liquid crystal display device. FIG. 6 shows such a discharge lamp lighting device, in which light rays emitted from a plurality (six in the figure) of cold cathode discharge lamps L1 to L6 are adapted to illuminate a liquid crystal display device by means of reflectors R and a light guiding plate PL disposed between the reflectors R. FIGS. 7 and 8 are block diagrams of conventional discharge lamp lighting devices described with reference to FIG. 6. Referring to FIG. 7, each lighting circuit LC comprises: a control circuit CT; a driving circuit D driven by the control circuit CT; a leakage transformer T; a discharge lamp L; and a resistor R1 connected in series to the discharge lamp L, and one lighting circuit LC is provided with each discharge lamp. Referring to FIG. 8, a lighting circuit LC comprises: a control circuit CT; a driving circuit D driven by the control circuit CT; a leakage transformer T; three ballast capacitors CB connected in parallel with one another; three discharge lamps L connected in parallel with one another and in series to respective ballast capacitors CB; and a resistor R1 connected in series to the three discharge lamps L.
The control circuit CT receives a DC power supply V, outputs a predetermined AC signal, detects a tube current flowing from the resistor R1 to the discharge lamps L, and controls the oscillation amplitude of the driving circuit D. In the discharge lamp lighting device shown in FIG. 7, one discharge lamp L requires one high-voltage and high-frequency transformer therefore requiring a plurality of transformers, and the plurality of transformers must be regulated so that tube currents in respective discharge lamps L are equal to one another. In the discharge lamp lighting device shown in FIG. 8, high-voltage and high-current capacitors are required, and the discharge lamps L have their lighting frequencies increased to, for example, 50 kHz for stable lighting operation. As a result, stray capacitances CS present between the reflectors R and the cold cathode discharge lamps L1 to L6 and present between the cold cathode discharge lamps L1 to L6 (see FIG. 6) make an impact, whereby tube currents in the discharge lamps change thus generating variance in illuminance.
To overcome the above problem, a discharge lamp lighting device shown in FIG. 9 for lighting a plurality of discharge lamps was disclosed in Japanese Patent Application Laid-open No. Hei 11-260580. The discharge lamp lighting device comprises: a DC power supply 31; and first and second switching elements (FETs) 12 and 13 which are connected in series to each other and which are connected respectively to one end and the other end of the DC power supply 31. A first series resonant circuit 15 consisting of an inductor 15a and a first capacitor 17 is connected to the connection between the first and second switching elements 12 and 13 and to the other end of the DC power supply 31. And a second capacitor 15b is connected to the connection between the inductor 15a and the first capacitor 17 and to the other end of the DC power supply 31. The discharge lamp lighting device further comprises: a second series circuit consisting of a first discharge lamp 18 and a first resistor 19; and a first control circuit 14 to control the switching frequencies of the first and second switching elements 12 and 13 in order to equalize the current in the first discharge lamp 18 to a predetermined value.
A second series resonant circuit 20 consisting of a variable inductor 21a and a third capacitor 22 is connected to the connection between the first and second switching elements 12 and 13 and to the other end of the DC power supply 31. A series circuit consisting of a fourth capacitor 10, a second discharge lamp 11, and a second resistor 23a to detect current is connected to the connection between the variable inductor 21a and the third capacitor 22 and to the other end of the DC power supply 31. A second control circuit 23 is provided which controls the inductance of the variable inductor 21a thereby equalizing the current in the second discharge lamp 11 to a predetermined value. For lighting a plurality of discharge lamps, there are provided a plurality of second series resonant circuits 20 each consisting of the variable inductor 21a and the third capacitor 22, a plurality of series circuits each consisting of the fourth capacitor 10, the second discharge lamp 11 and the second resistor 23a to detect current, and plurality of second control circuits 23.
The FETs 12 and 13 as switching elements are alternately switched on and off by respective control signals supplied from the first control circuit 14 comprising a microcomputer, and so on to respective gates of the FETs. The first control circuit 14 is capable of controlling the frequency of the control signal across a predetermined range. The connection between a source S of the FET 12 and a drain D of the FET 13 is connected to a cathode of the DC power supply 31 via the series circuit consisting of the inductor 15a constituting the first series resonant circuit 15 and the second capacitor 15b, and the inductance of the inductor 15a and the capacitance of the capacitor 15b are set to respective predetermined values so as to set a resonant frequency f0 of the first series resonant circuit 15 to a predetermined frequency.
The above discharge lamp lighting devices have the following problem. Since the inductance value of the variable capacitor 21a is controlled so that the current of the second discharge lamp 11 is equal to a predetermined value, the second control circuit 23 for controlling the inductance value is required. Further, for lighting a plurality of discharge lamps, there must be provided a plurality of second series resonant circuits 20 each consisting of the variable inductor 21a and the third capacitor 22, a plurality of series circuits each consisting of the fourth capacitor 10, the second discharge lamp 11 and the second resistor 23a to detect current, and plurality of second control circuits 23. Accordingly, for example, if six discharge lamps are lighted as shown in FIG. 6, its circuit has to be complicated and the number of the components is inevitably increased, thereby making it difficult to realize cost reduction. Also, the increased number of the components tends to degrade the reliability of the device.
The present invention has been made in light of the above problem, and it is an object of the present invention to provide a reliable discharge lamp lighting device, which uses a limited number of components, and which is capable of lighting a plurality of discharge lamps without suffering the effects of stray capacitances present between and around the discharge lamps.
In order to achieve the above object, according to a first aspect of the present invention, a discharge lamp lighting device comprises a plurality of discharge lamps, at least one reflector to reflect light rays emitted from the discharge lamps, and at least one leakage transformer, and each leakage transformer is adapted to light three discharge lamps, and comprises: a first leakage transformer, which has two primary windings and two secondary windings structurally independent of the two primary windings, and is adapted to light two discharge lamps of the three; and a second leakage transformer, which has a primary winding and a secondary winding structurally independent of the primary winding, and is adapted to light remaining one discharge lamp of the three.
According to a second aspect of the present invention, in the discharge lamp lighting device of the first aspect, the plurality of discharge lamps are disposed in parallel with one another, and the one discharge lamp lighted by the second leakage transformer is located between the two discharge lamps lighted by the first leakage transformer.
According to a third aspect of the present invention, in the discharge lamp lighting device of the first aspect, the first and second leakage transformers are driven by the same driving circuit, and three discharge lamps are lighted in-phase with one another.
According to a fourth aspect of the present invention, in the discharge lamp lighting device of the first aspect, the numbers of turns on the primary and secondary windings of the second leakage transformer are determined so as to equalize respective currents flowing in the three discharge lamps when the discharge lamps are lighted.
According to a fifth aspect of the present invention, in the discharge lamp lighting device of any one of the first to fourth aspects, the numbers of turns on the primary windings of the first leakage transformer are equal to each other and the numbers of turns on the secondary windings of the first leakage transformer are equal to each other.
According to a sixth aspect of the present invention, in the discharge lamp lighting device of the first aspect, the first leakage transformer comprises: a frame-core shaped substantially rectangular; and two bar-cores disposed parallel to each other and orthogonal to two opposing sides of the frame-core with a predetermined gap from the frame-core, and each bar-core having a primary winding and a secondary winding structurally independent of the primary winding, and the second leakage transformer comprises: a frame-core shaped substantially like square-U letter; and a bar-core disposed orthogonal to two opposing sides of the frame-core with a predetermined gap from the frame-core, and having a primary winding and a secondary winding structurally independent of the primary winding.
According to a seventh aspect of the present invention, in the discharge lamp lighting device of the first aspect, the first leakage transformer comprises: a frame-core shaped substantially rectangular; and two bar-cores disposed parallel to each other and orthogonal to two opposing sides of the frame-core with a predetermined gap from the frame-core, and each (bar-core) having a primary winding and a secondary winding structurally independent of the primary winding, and the second leakage transformer comprises: a frame-core shaped substantially rectangular; and a bar-core disposed orthogonal to two opposing sides of the frame-core with a predetermined gap from the frame-core, and having a primary winding and a secondary winding structurally independent of the primary winding.
Accordingly, the discharge lamp lighting device of the present invention can be provided, which can be produced with a limited number of components, at a low cost, with a high reliability, and which can light a plurality of discharge lamps without suffering the influence of stray capacitances present between and around the discharge lamps.